1. Field of the Invention
A device for converting the rate of a pulse train which is proportional to the rotational velocity of the vehicle wheel into a binary number.
2. Prior Art
In any accurate anti-skid control system there always exists the problem of converting the rotational velocity of a wheel which is measured as a pulse train into a signal useful for controlling the brake pressure. The rate of the pulse train is proportional to the rotational velocity. As a rule, this method involves a computing process having many steps and the inaccuracies occurring in prior art devices using analog elements are often relatively high, so that these analog devices cannot fully solve the problem. Furthermore, analog modules are in general relatively expensive. Therefore, anti-skid control have already been devised for operation of digital computers. For example an anti-skid control device is disclosed in U.S. Pat. No. 3,746,925, assigned to the same assignee of this application. In the device therein disclosed the pulses arriving from a wheel sensor are counted over two successive like intervals and compared with one another so that the difference between the number of pulses counted for successive intervals is indicative of the acceleration of the wheel. It will be obvious from this that the interval must be substantially longer than a period of the slowest pulse rate of this pulse train. Therefore, there is a relatively long time before a value indicating the rotational acceleration of the wheel will be available. For that reason alone a quick responding and thus accurate anti-skid control is not possible.
In U.S. Pat. No. 3,805,089, a device of the type previously referred to is disclosed in which each pulse arriving from the wheel sensor corrects a binary number which is representative of the rotational velocity. This is disclosed as being achieved in that initially when a pulse arrives, the contents of a first register are compared to a predetermined value. The difference between the two values is transferred to a second register which adds this difference to its content, taking into consideration the sign. The content of the second register are then read into the first register at a constant high rate, in other words added to the value already held in the first register. When comparing the contents of the second register with the predetermined value, the second register is simultaneously reset to zero. It results from such a feedback arrangement of the difference determined by the comparator through the two registers that this difference is indicative of the rotational acceleration of the wheel. Since the second register operates in the manner of an integrator, its binary contents represent the instantaneous wheel velocity. Consequently, this arrangement provides at all times a binary number representative of the rotational velocity which is corrected upon the arrival of each new pulse.
However, in the arrangement of U.S. Pat. No. 3,805,089, there are decisive disadvantages. In the event of a sudden wheel lock-up the pulse rate will go down to zero so that no signal will appear at the comparator to trigger the comparison function. In this case it will not be possible to determine the difference between the contents of the first register and the predetermined value so that the contents of the second register cannot be corrected. The second register would thus continue to deliver to the connected computer a signal indicative of the immediately preceding rotational velocity of the wheel, although the wheel has stopped rotating. At the same time the contents of the first register would continue to be incremented until it overflows. Accordingly, the device disclosed in U.S. Pat. No. 3,805,089 does not provide an accurate means to determine the rotational velocity of the wheel. In other words, the contents of the second register for a subsequent anti-skid control or the like but only provides means to average the difference obtained by the comparator and representing the wheel deceleration by means of a third register over a predetermined interval and to use it further. Thus, this disclosed prior art device does not permit the use of the signal which normally indicates the rotational velocity of the wheel because of the above-described case, which is just the critical case in anti-skid control systems.